1. Field of the Invention
The present invention relates to semiconductor optical devices, and more particularly, to a technology for improving the transport characteristics of carriers at heterointerfaces.
2. Description of the Related Art
Recently, the demands for greater transmission capacities are rapidly growing with factors such as the explosive increase in the Internet users. The transmission speeds of Gbps levels, that have traditionally been used for trunk-system optical communications networks, should be necessary after 5-10 years for the networks relatively short in transmission distance such as LAN or MAN. It will be essential that, in addition to the high-speed modulation characteristics mentioned above, the optical modules used for these networks should be supplied at low costs in consideration of use by a large number of users. Therefore, semiconductor lasers which have superior high-temperature lasing characteristics are considered to be suitable for such modules.
Conventional semiconductor lasers, modulators, and other devices for optical communications have fabricated mainly by using GaInAsP on an InP substrate. This quaternary material consists of four kinds of main constituent elements so the design flexibility both in bandgap and in lattice constant is high. In the wavelength compositions of the 1.30-μm and 1.55-μm bands adapted to optical communications, therefore, desired lattice strain can be easily introduced into quantum well active layers, whereby higher device performance has been achieved. However, since the energy difference in the conduction band (ΔEc) between the quantum well layer and barrier layer of the quantum well in GaInAsP active layers is small, the confinement of electrons is weak and the deterioration of the device characteristics at high temperature is remarkable. For the same physical reason, the active layer has small gain and low relaxation oscillation frequency. From these reasons, the semiconductor lasers that use GaInAsP are probably difficult to satisfy the future demands for speeding-up and cost reduction.
In contrast to this, the use of the AlGaInAs as an active layers on an InP substrate has been intensively investigated by many groups to improve the characteristics of the semiconductor lasers for optical communications in recent years. Compared with the quantum well active layers using GaInAsP, those using AlGaInAs have larger ΔEc and can confine electrons effectively even at high temperature. In addition, the gain in active layers using this material is large, so that semiconductor lasers with better high-temperature and high-speed modulation characteristics can be obtained by using this material. Furthermore, it is reported that these physical advantages also bring about the improved extinction characteristics of electro-absorption optical modulators using AlGaInAs.
An example of an energy band diagram of typical AlGaInAs-based optical devices is shown in FIG. 1. The upper line in this figure denotes the energy position of a conduction band, and the lower line denotes the energy position of a valence band. The distance between both lines is equivalent to the bandgap in each layers. As can be seen in the figure, for example, InP is used as cladding layers even in AlGaInAs-based optical devices. So semiconductor layers of conventional GaInAsP-based materials are introduced into the appropriate part. Thus, the device designing and manufacturing technologies hitherto accumulated for GaInAsP-based devices have been applied to appropriate part to achieve excellent performance in AlGaInAs-based optical devices.
Another example using a similar technology is a semiconductor laser using GaInNAs as an active layers on a GaAs substrate. GaInNAs, a semiconductor material developed in recent years, can offer the active layers having even larger ΔEc than that by using AlGaInAs. Whereas GaInNAs is expected as a very promising material for further improvement of high-speed modulation characteristics of semiconductor lasers, this material has the problem in its crystal quality. The emission efficiency of GaInNAs decreases with N-content. For this reason, in order to obtain better lasing characteristics at a 1.3-μm band, the high In-content and small N-content are adopted in present GaInNAs active layers. As a result, GaInNAs layers with high compressive strain as large as nearly +2% are used as quantum well layers. This large lattice strain is most likely to deteriorate the performance and long-term reliability of the device. An attempt is therefore being studied that a strain compensation structure by applying the GaNPAs (or the like) that has tensile strain as an barrier layers to reduce the average strain of the entire device.
The two examples described above are common in that a stacked structure with one III-V alloy semiconductor layers containing at least As (arsenic) as the group-V element, and another III-V alloy semiconductor layers containing at least atoms different from As, such as N (nitrogen) or P (phosphorus), as the group-V element, is introduced to realize better device characteristics. It goes without saying that this stacked structure with different kind of group-V atom is also applicable to many other types of devices not described here and can contribute to the improvement of device characteristics. In addition, although only optical devices such as semiconductor lasers and optical modulators have been described as examples, these examples do not limit the applicable kinds of optical devices, provided that the stacked structure is applied to many other types of the semiconductor devices.